The present invention relates to a retroreflective sheet for use on traffic signs, which enables safe utilization of roads at night, or labels, stickers, emblems or the like for vehicles or decorative purposes. More particularly, the present invention relates to a sealed lens type retroreflective sheet which is suitable for co-molding by injection molding, blow molding or the like wherein a retroreflective sheet can be placed within a mold for a curved substrate and molded with a resin to form a molded product having a curved surface, and, after molding, the sheet retains a significant portion of its reflection luminance as compared with the reflection luminance value before molding. That is, the present invention relates to a sealed lens type retroreflective sheet which, at the time of molding, enables a resin with the retroreflective sheet mounted thereon to be stretched to a desired form without undesirable deterioration in its optical properties.
In conventional sealed lens type retroreflective sheet, a transparent microspherical lens is independently embedded in a transparent anchoring layer, and a metallic reflective layer is provided on the back side of the microspherical lens through a focal resin layer. In these sealed lens type retroreflective sheet, since a microspherical lens is embedded in a thin sheet material, problems occur such as embrittlement and blister. Further, such a retroreflective sheet is unsuitable for co-molding wherein the retroreflective sheet is molded together with a substrate on which the retroreflective sheet is to be laminated. For this reason, Japanese Unexamined Patent Publication (Kokai) No. U.S. Pat. No. 4,983,436 (Bailey et al.) disclose that a single backing film layer is provided to reinforce the sheet, thereby solving the embrittlement problem. On the other hand, in Japanese Unexamined Patent Publication (Kokai) No. 59-5051, stretching is carried out so as to cause cracking in a metallic reflective layer to prepare an intermediate product having discontinuous portions, and, thereafter, the other layers are formed. That is, the blister problem is solved by improving the air permeability.
Mounting of a sealed lens type retroreflective sheet on a flat substrate has hitherto been performed by contact-bonding the sheet to the substrate by taking advantage of a pressure-sensitive adhesive provided on the surface of the sheet remote from the reflecting surface. Further, in the case of mounting of a retroreflective sheet on an uneven surface like a gentle quadratic surface, contact bonding has been used as with the case of the flat substrate. In these cases, the retroreflectivity of the retroreflective sheet can be maintained after mounting because the retroreflective sheet per se is not substantially stretched by the mounting operation. This method, however, is unsuitable for mounting on a quadratic surface having sharp irregularities because the sheet cannot be exactly fitted to such a surface without leaving any space. For this reason, a co-molding method is used wherein the retroreflective sheet is mounted when the resin is molded by injection molding or blow molding. In this case, the retroreflective sheet is stretched. The stretching of the sheet results in stretching of the focal resin layer. This means that the resin constituting the focal resin layer, for example, polyvinyl butyral, is also stretched. The stretching of the focal resin layer causes the thickness of the resin layer to gradually decrease with an increase in percentage stretching, making the luminance of the sheet decrease. Further stretching thereof causes large cracks in the focal resin layer, making it impossible to maintain the appearance as before the molding.
The present invention provides, in one aspect, a sealed lens type retroreflective sheet having good appearance, which is suitable for co-molding by injection molding, blow molding or the like wherein the retroreflective sheet can be laid within a mold for a substrate, the sheet and a molding resin can be combined to form a molded product, and, after molding, the sheet retains a significant portion of its reflection luminance as compared with the reflection luminance value before molding.
A preferred sealed lens retroreflective sheet of the invention can be used for co-molding, and comprises a metallic reflective layer, a microspherical lens layer disposed as a single layer, and a transparent focal resin layer disposed between the reflective layer and the microspherical lens layer, characterized in that the focal resin layer comprises a mixture of cellulose ester and polyvinyl butyral resin. Preferably, the focal resin layer consists essentially of a cellulose ester and a polyvinyl butyral resin.